1. Field of the Invention
The present invention relates hybrid electric vehicles having a high voltage bus and a low voltage bus, and in particular, to transferring energy from the high voltage bus to the low voltage bus.
2. Background Art
Many hybrid electric vehicles, such as series, parallel, and parallel-series hybrids, typically include a high voltage bus and a low voltage bus. Typically, the high voltage bus transfers energy between components used to drive the vehicle and the low voltage bus transfers energy to accessory loads.
The high voltage bus can be electrically coupled to the low voltage bus by a DC/DC converter, allowing energy to be transferred between the buses. Because the high voltage bus maintains a high voltage, it is typically discharged when the vehicle is not in operation. In doing so, a battery or other high voltage energy storage device coupled to the high voltage bus is isolated from the rest of high voltage bus.
Commonly, the isolation is achieved by opening contactors used to electrically couple the high voltage energy storage device to the rest of the high voltage bus. The open contactors must then be closed to start the vehicle. Prior to closing the opened contactors, energy is transferred from the high voltage energy storage device to the high voltage bus for precharging the high voltage bus.
The precharging prevents an instantaneous short from occurring when the contactors eventually close. The precharging ends when the high voltage bus is sufficiently charged and the contractors close.
After precharging, but before the entire powertrain is enabled, the system waits in a state where the high voltage bus is enabled but a primary energy source, typically an internal combustion engine or a fuel cell system, is temporarily disabled. This is referred to as “prestart.”
During such prestart, experimental testing indicates some degradation can occur to a low voltage battery coupled to the low voltage bus. In particular, the degradation can occur when the accessory loads powered by the low voltage bus are operated during extended periods of prestart, such as when the ignition key is in the run position and the lights or radio are on. Accordingly, there exists a need to limit such degradation to the low voltage battery.